NZ First have a clause in their party constitution that tries to enable the party to fine any list or electorate MP who resigns or is expelled  $300,000 at the discretion of the Board.

NEW ZEALAND FIRST PARTY CONSTITUTION 2013

57 Parliamentary Division

(g) Upon a member of the Parliamentary Division ceasing to be a member of the Parliamentary Division because he/she has resigned from or has been expelled from the New Zealand First parliamentary caucus, or has ceased to be a member of the Party, then whether the member is a constituency member of parliament or a list member of parliament, he/she must resign his/her parliamentary seat as soon as practicable and in any event not later than 3 days after the date of cessation.

(h) In order to provide the means for enforcement of the preceding article 57(g) concerning the obligation of a member or former member to resign his/her parliamentary seat, and as a condition precedent to selection as a New Zealand First parliamentary candidate, and in consideration of selection as such, every member who agrees to become a candidate, and every member who stands as a candidate, and every member who is elected as a New Zealand First list member of parliament or as a New Zealand First constituency member of parliament, before being selected as a candidate, before standing as a candidate, before election as a New Zealand First member of parliament, and before accepting his/her seat in parliament and before being sworn in as such, and at any other time when required by the Board to do so whether before or after election and whether before or after being sworn in as a member of parliament, shall agree to give and shall sign a written undertaking, intended to be a legally enforceable contract (the resignation obligation contract) , under which he/she agrees to uphold observe and perform all of the provisions of this article 57 of this constitution and its amendments, and in particular to a fundamental term of the contract which will be the essence of  the contract, and which will impose a liability for liquidated damages in the sum of $300,000 (three hundred thousand dollars) for any breach of article 57(g) of this constitution and its amendments, concerning the obligation of a member or former member to resign his/her parliamentary seat, if he/she ceases by any means and for any reason and in any circumstances whatsoever to be a member of the Parliamentary Division during the term for which he/she has been elected. The Board may however compromise the amount of liquidated damages payable or waive the imposition of liability for liquidated damages in its sole and unfettered discretion without having to have any reason for doing so, without having to give any reason for doing so, and without being under a any obligation to do so or to consider fairness, natural justice, or any other consideration whatsoever;
and the Board shall not enforce the resignation obligation contract under this article 57(h) at any time that legislation exists which requires or determines that a member of Parliament to resign or relinquish his/her parliamentary seat upon the grounds contained in or similar to those specified in article 57(g).

NZ First  Party rules:  nz_first_constitution_nov_2013.pdf

Law professor Andrew Geddis takes it apart: I’m right, Winston’s not, so there

The $300,000 figure here clearly is designed to present an MP who leaves NZ First with Hobson’s choice. Either quit as an MP, or face ruinous financial consequences. And because the rule has this effect – it is designed to force an MP from Parliament – I don’t think it will be enforceable in court. And a rule of this nature only has teeth if there is a court that is prepared to, as a matter of law, make someone actually pay up the penalty figure.